A generator is a device which converts mechanical energy into electrical energy. The source of this mechanical energy can be water turbine, wind turbine, internal combustion engine, steam turbines, gas turbines etc. On the basis of the type of electricity produced by generators, the generators are of two types. Firstly, AC generator, secondly, DC generator.

AC Generator

An AC generator is an electric generator that converts mechanical energy into electrical energy in the form of alternating current.

The direction of the current generated by AC generators changes periodically, therefore the name alternating current.

Parts of AC Generator and Their Functions

ac generator diagram

AC generator primarily consists of : 

Armature

This component primarily consists of coils of wire that carry the current of the generator. 

Function:

Armature is responsible for rotation. When the armature rotates between the poles of the magnet about an axis perpendicular to the magnetic field, flux linked with the armature changes.

Magnetic flux = ΦB = BA cos θ

Thus, ΦB = BA cos ωt

Here, ω is angular speed. θ is the angle between the magnetic field and vector B. A is area of coil.

This change in the flux induces an emf in the armature which produces an electric current.

Field magnet

The field consists of coils of conductors which receive a voltage from the source and produce magnetic flux.

Function:

The magnetic flux in the field induces an emf in the armature.

Slip Rings

Slip rings are electric connection of the generator. 

Function:

To conduct the current from the stationary part to the rotating part.

Prime Mover

The source of the power ,i.e., engine, turbine motor etc. 

Function:

Uses to drive the generators 

Rotor

The rotating part of the generator.

Function:

Production of a magnetic field in the generator. 

Stator

The stationary part of an generator.

Function:

converts the rotating magnetic field to electric current.

Principle of AC Generator

AC generators work on the principle of Faraday’s law of electromagnetic induction.

According to this law, “Whenever a closed loop conductor is placed in a varying magnetic field, due to change in the flux, an electromotive force gets induced across the conductor .”

The following are the ways to produce varied magnetic field: 

  1. moving the coil
  2. moving magnet
  3. rotating the coil relative to magnetic field

An ac generator uses third way from the above three.

Construction of AC Generator

AC generator consists of two poles of a magnet to provide a magnetic field. There is a coil of rectangular shape which placed in between poles of magnet known as armature. The armature connects with two slip rings. Slip rings are electric connection of the generator which conduct current from rotating part to stationary part. These slip rings attach to carbon brushes.

The rectangular coil is capable of rotating about an axis which is perpendicular to the magnetic field which further induces emf.

construction of ac generator

Working of AC Generator

When the armature placed between two magnetic poles rotates upon an axis which is perpendicular to the magnetic field, the magnetic flux changes continuously.

The flux at time t is :

ΦB = BA cos θ = BA cos ωt

Now according to Faraday’s law, due to changes in flux, an emf will be induced in the armature as :

ε = -N dΦB/dt 

Thus, ε = -NBA d(cos ωt) / dt 

Hence, ε = -NBAωsin(ωt)

Therefore, ε = ε₀ sin(ωt)

This induced emf produces electric current through the galvanometer and the slip rings and brushes. 

An alternating emf is generated by a loop of wire rotating in a magnetic field : 

working of ac generator

Uses of AC generator

  • Hydro-electric generators: These generators are uses in producing electrical energy from mechanical energy of water falling from a height ie. from dams.
  • Marine alternators: This type of AC generators generate electric energy in marine applications.
  • Thermal generators: Thermal generators produce electrical energy from mechanical energy of steam. Water is heated to produce steam using coal or other sources. This steam at high pressure produce the rotation of the armature.
  • Automotive alternators: In automobiles, ac generators are known as alternators. All the electric power in the vehicles including charging of battery comes in using AC generators.
  • Power generators : Nuclear reactor heats up water to produce steam. This further produce the rotation of armature finally producing electrical energy.
  • In power generation from windmills

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